1. Field of the Invention
The invention relates to the field of reducing by means of a catalyst primary injurious substances generated in internal combustion engines. Within the scope of such a primary injurious substance-reducing system the invention relates in particular to a converter for a device for introducing ammonia (NH3) into the exhaust gas flow of an internal combustion engine, comprising a pressure-tight reaction receptacle for generating NH3 by heating a substance which thermolytically separates NH3 in the presence of heat and a heating unit for heating the reaction receptable operating temperature for generating the NH3. The invention relates further to a process for filling such a converter while it is at its operating temperature with new, unconsumed substance splitting off NH3.
2. Description of the Prior Art
Apart from carbon monoxide (CO) and hydrocarbons (HC), in particular the nitric oxides (NOx) are among the environmentally harmful, directly emitted, primary injurious substances which are generated during the operation of internal combustion engines, in particular Diesel engines. The use of three-way catalysts, such as are used in Otto engines and gas engines, cannot be used in the exhaust of Diesel engines due to the presence of an oxygen excess. For this reason, for the reduction of the nitric oxide emission in Diesel engines a selectively operating SCR catalyst (Selective Catalytic Reduction Catalyst) has been developed in which, in the presence of an added reducing agent, namely ammonia (NH3), the expelled nitric oxides are reduced to N2 and H2O.
A device according to the species for introducing ammonia into the exhaust gas flow of an internal combustion engine of a motor vehicle is known from DE 197 20 209 C1 by the applicant. Such a denoxing unit comprises a converter in which is disposed a substance thermolytically separating NH3 or a substance mixture thermolytically separating NH3. The substance can be ammonium carbamate, for example. The converter is connected with the exhaust gas line of a Diesel engine via a feed line with the feed line terminating in the exhaust gas line before the input side of an SCR catalyst. A clocked valve is provides as a dosing device which is driven by a control unit such that, as a function of specific motor operation characteristics, the requisite quantity of NH3 can be injected into the exhaust gas flow. The converter substantially comprises a pressure-tight reaction vessel encompassed by a heating device implemented as a heating coil. The heating device is integrated into the cooling water circulation of the Diesel engine via an inlet conduit and a drain conduit.
By heating the ammonium carbamate which is used, for example, as the substance thermolytically separating NH3, it is broken down into NH3 and CO2. This gas mixture collects in the pressure-tight reaction vessel until the build-up of a corresponding internal pressure. When a specific internal pressure has been reached in the reaction vessel, an equilibrium state is established such that no further ammonium carbamate is broken down. Under operating conditions of the engine, in which the cooling water flowing through the heating device, as a rule, has a temperature between 80 and 100xc2x0 C., in the reaction vessel a pressure obtains corresponding to the state of equilibrium, which for ammonium carbamate is approximately 8 bar. In order for a sufficient quantity of NH3 to be provided,for injection into the exhaust gas flow through the converter, it must store a specific minimum quantity of substance thermolytically separating NH3 or NH3 separated from it. For this reason the device cannot be operated for a length of time sufficient for the entire substance thermolytically separating NH3 to be gasified and the internal pressure in the reaction vessel has in this way is degraded. Opening the reaction vessel in this state for replenishing it with new, unconsumed substance, thermolytically separating NH3, cannot readily take place due to the internal pressure. Even if one were to wait until the converter has cooled down again to ambient temperatures and thereby the internal pressure has also been lowered in order to open the reaction vessel, a certain residual pressure remains in the converter especially if the ambient temperatures, such as for example during the summer, are relatively high. Moreover, such a cooling requires too much time and cannot be carried out within the scope of normal fueling of the motor vehicle.
Building on the discussed prior art, the invention is therefore based, for one, on the task of further developing a converter according to the species for a device for supplying ammonia into the exhaust gas flow of a combustion engine such that it can readily be filled with new substance which thermolytically separates NH3 at its normal operating temperatures and within the time frame of normal motor vehicle fueling.
The invention is further based on, the task of providing a method for filling such a converter when it is at its operating temperature with new, unconsumed substance which thermolytically separates NH3.
The first-mentioned task is solved according to the invention thereby that the reaction vessel comprises an outlet, provided with connection means and an inlet, also provided with connection means and spaced apart from the outlet, with which connection means the interior of the reaction vessel can be connected to a filling and withdrawal unit of a carrier medium disposed in the reaction vessel and for filling the reaction vessel with new substance which thermolytically separates NH3, transported by the carrier medium.
The second-mentioned task is solved according to the invention through a method for filling a converter containing a substance at operating temperature for a device for supplying NH3 into the exhaust gas flow of a combustion engine, comprising a pressure-tight reaction vessel containing the substance which thermolytically separates NH3, a heating unit for heating the reaction vessel to operating temperature and means for supplying the separated NH3 into the exhaust gas flow of the combustion engine, comprising the following steps:
Connecting the reaction vessel to a filling and withdrawal unit, with the interior of the reaction vessel being brought into connection with the filling unit via an inlet and with the withdrawal unit via an outlet,
Filling of the reaction vessel with new substance which thermolytically separates NH3 using a liquid or gaseous carrier medium transporting the substance thermolytically separating NH3, which neither dissolves nor chemically binds the NH3 separated from the substance thermolytically separating NH3, which carrier medium is introduced through the inlet into the reaction vessel with the simultaneous drawing off of a quanity of the carrier medium through the outlet after a separation of the substance which thermolytically separates NH3 from the quantity of carrier medium to be drawn off, and
After filling the reaction vessel, separating the reaction vessel from the filling and withdrawal unit.
In the case of the converter according to the invention it is provided to structure the reaction vessel with an inlet and with an outlet with the outlet usefully being disposed in the region of the bottom of the reaction vessel. The inlet as well as also the outlet are provided with connection means which permit a connection of the inlet and of the outlet to a filling and withdrawal unit. Usefully the filling and withdrawal unit is disposed stationarilyxe2x80x94such as the gasoline pump of a filling station. A converter connectable in this manner to a filling and withdrawal unit permits the use of a;carrier medium for filling the converter with new, unconsumed substance thermolytically separating NH3, with this substance being introduced into the reaction vessel through the carrier medium via the inlet. Simultaneously with the introduction of the new substance, carrier medium is drawn from the outlet which, before it is drawn off from the outlet, had been separated from the substance which thermolytically separates NH3 entrained by the carrier medium.
Due to the possibility of connecting the reaction vessel to a filling and withdrawal unit, by introducing a carrier medium into the reaction vessel, the latter can be filled with new substance thermolytically separating NH3, without in principle a preceding pressure degradation being necessary for this purpose. The filling of the reaction vessel can thus also be carried out while substantially maintaining the internal pressure obtaining in the reaction vessel.
In the case of such a converter, further, the possibility exists that it can be integrated into a cooling means circulation in which, via the inlet, coolant can be introduced into the reaction vessel and be drawn off from the outlet in the heated state after it has flown through the reaction vessel. This circulation can be formed by the filling and withdrawal unit such that after sufficient cooling of the reaction vessel, new substance thermolytically separating NH3 can be introduced with the carrier medium into the reaction vessel. It is therein useful that the coolant and the carrier medium are identical. With this type of refilling of the converter, the reaction vessel also does not need to be opened.
An advantageous embodiment provides that the coolant and the carrier medium are an identical fluid, for example oil. The use of a liquid coolant is useful since through the higher heat capacity of a liquid compared to a gas, a more rapid cooling of the reaction vessel can be achieved.
By providing a liquid carrier mediumxe2x80x94a carrier liquidxe2x80x94it is provided that in the reaction vessel, after it has been filled with new substance splitting off NH3, a specific quantity of carrier liquid remains in the reaction vessel. Heating the substance thermolytically separating NH3, which is disposed within the carrier liquid, for example in the form of small spheres or also as a suspension, takes place very much more rapidly and more uniformly since the heating of the interior of the reaction vessel does not take place through heat radiation but substantially through heat conduction.
If it is provided that the reaction vessel is newly filled without preceding cooling and concomitant pressure degradation, the connection means of the inlet or of the outlet are equipped with corresponding auxiliary means. Such auxiliary means, for the inlet can be a one-way valve and for the outlet a pressure reducing valve whose high-pressure side is disposed such that it is directed toward the reaction vessel.
When using a liquid as the carrier medium, a separation of the thermolytically separating NH3 from the carrier liquid takes place, for example, by connecting in series a filter before the outlet. The substance, for example ammonium carbamate, thermolytically separating NH3 is flushed with the carrier liquid, for example an oil, into the reaction vessel in the form of small spheres or pellets and remains in the reaction vessel at a predetermined level of the liquid. It can be provided that between the provided liquid level of the carrier liquid with the substance thermolytically separating NH3 and a superjacent reaction vessel region, in which the thermolytically separates reaction gas, namely NH3 or an NH3 gas mixture accumulates, a membrane permeable to NH3 is disposed. Such a division, moreover, has the advantage that the membrane acts in the manner of a baffle such that the liquid in the reaction vessel is held quasi-stationarily in the reaction vessel.
As the carrier medium such gaseous or also liquid.substances or,mixtures of substances can be provided which do not dissolve thermolytically separated NH3 or bind it chemically in other ways. Further, the carrier medium used should also not be able to dissolve the substance thermolytically separating NH3. However, it can be provided that the carrier medium binds the further substances or substance mixtures formed in the thermolytic NH3 cleavage. In this way, pure NH3 can be drawn off at the output side of the converter for supplying it into the exhaust gas flow of the combustion engine.